1. Field of the Invention
The present invention relates generally to an electrically powered vehicle, such as an electric vehicle (EV), a hybrid electric vehicle (HEV) or a fuel cell vehicle (FCV). More specifically the invention relates to a strategy to diagnose a potential fault in an electric motor. The present invention an improve the robustness of operation and diagnose potential faults in electric motors by utilizing a sensorless control scheme augmented by feedback from a low-resolution position and speed sensor.
2. Background of the Invention
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a parallel/series “split” configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier gear. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or driveability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shut down. Nevertheless, new ways must be developed to optimize the HEV's potential benefits.
One such area of development is diagnosing potential faults in an electric motor and increasing the robustness of the operation of an electric motor. An effective and successful HEV design (or any vehicle propelled by electric motors) requires reliable operation. Reliable operation can be improved through careful diagnosis of potential faults within the electric motor and increasing the robustness of electric motor operation. Thus there is a need for a strategy to effectively diagnose potential faults in an electric motor propelled vehicle's electrical motor and increase the robustness of electric motor operation. One strategy to improve the robustness of operation and diagnose potential faults in electric motors is to utilize a sensorless control scheme coupled with feedback from a low-resolution position and speed sensor.
Sensorless control schemes for electric machines (also referred to as electric motors or generators) are known in the art. Electric machines can be induction, synchronous or switched reluctance type. For example, U.S. Pat. No. 6,137,258 to Jansen describes a system for speed-sensorless control of an induction machine (electric motor) that includes a flux regulator and torque current calculator for operating the machine in a saturated state. U.S. Pat. No. 6,163,119 to Jeong describes a sensorless speed control method for a high speed motor that utilizes a reverse electromotive force. U.S. Pat. No. 5,920,175 to Jones et al. describes a sensorless control system for operating an inverter coupled to a switched reluctance machine that includes an instantaneous position generation circuit that develops a signal for controlling commutation of the switched reluctance machine. See also, U.S. Pat. No. 5,811,957 to Bose et al., and U.S. Pat. No. 6,104,113 to Beifus.
Low resolution shaft position and speed sensors are also known in the art and are commonly installed in automotive vehicles. Crankshaft position and speed sensors, camshaft position and speed sensors and transmission position and speed sensors are examples of low resolution shaft position sensors used in automotive vehicles.
However, sensorless control schemes for electric motors and low resolution position and speed sensors each have their drawbacks and limitations. Sensorless control schemes often fail at low shaft rotational speeds and thus often limited to high shaft rotational speeds. Low resolution shaft position and speed sensors can measure shaft position and speed at low shaft rotational speeds, but have limited accuracy.